The demand for improved performance of automatic transmission fluids spurs a continuing search for new additives and lubricating oil formulations to achieve the desired goals. An automatic transmission is a complex hydraulic mechanism having friction sensitive power transmitting means which incorporates the function of a torque converter, wet clutches and planetary gearing in a relatively compact scale unit. The device requires a transmission fluid which provides lubricity, extreme pressure, dispersant and antiwear properties as well as carefully selected frictional properties. In addition, the fluid must not be corrosive to copper alloys or any way deleterious to the synthetic seals and transmission. Another requirement for the fluid is that it maintain good lubricity and friction modifying properties under prolonged high shear, high temperature conditions encountered in this environment.
Carboxylic acids and their derivatives are widely employed as lubricity agents or friction modifiers in mineral base automatic transmission fluids. These lubricity agents provide commercial automatic transmission fluids having good service life. However, the industry is continually searching for friction modifying additives which have improved stability properties and antiwear properties and thereby increase the operational life of the automatic transmission fluid. A breakdown in stability of the friction modifier is indicated by rising coefficient of friction in the transmission fluid and by an early onset of erratic harsh shifting in service and in automatic transmission tests.
One such improved automatic transmission fluid of superior stability is described in U.S. Pat. No. 3,879,306 having improved friction modifier stability which contain as a friction modifier N-(hydroxyalkyl) alkenyl succinamic acid characterized by the formula: ##STR2## or a mixture of said succinamic acid and N-(hydroxyalkyl) alkenylsuccinimide characterized by the formula: ##STR3## where R.sup.2 is alkenyl of from 10 to 30 carbons and A is a divalent saturated aliphatic hydrocarbon radical of from 1 to 6 carbons.